The present invention relates to linear motors and, more particularly, to a magnet configuration for a linear motor.
In one linear motor, commutator contacts are pendant from the stage. The contacts contact one or more power rails, and one or more coil contacts. As the stage moves along the armature, the location of the stage, relative to the armature is automatically accounted for by applying power to the stationary armature windings through the commutator contacts.
In other linear motors, it is conventional to employ a service loop of wires between the moving stage and the stationary elements. The location of the stage is updated using a magnetic or optical position encoder on the stage which senses markings on an encoder tape stationary alongside the path of the stage. The location is connected on the service loop to a stationary motor controller.
Generally, the important location information is the phase of the stage relative to the phase of the armature. For example, in a three-phase armature, the windings are disposed in repeating sets of three for phases A, B and C. If the center of the A phase winding is arbitrarily defined as 0 degrees, then the centers of the B and C windings are defined as 120 and 240. There may be two, three or more sets of windings as required for the travel distance of the stage. Normally, all A phase windings are connected in parallel. The same is true of all B and C phase windings. Thus, when the location of the stage requires a certain voltage configuration on the particular windings within the influence of the magnets on the stage, besides powering these windings, all of the other windings in the armature are also powered. The maximum force obtainable from a linear motor is limited by the allowable temperature rise in the armature windings. When all windings are powered, whether they contribute to motor force or not, more armature heating occurs than is strictly necessary for performing the motor functions.
Some linear motors in the prior art have responded to this heating problem using switches that are closed only to the armature windings actually within the influence of the magnets.
The need for a cable loop connecting moving and stationary elements is inconvenient, and limits the flexibility with which a system can be designed. The wiring harness requires additional clearance from the linear motor to prevent entanglement between the motor and any equipment or items that may be adjacent to the linear motor path. In addition, the wiring harness adds additional weight to the moving element of the linear motor. Furthermore, manufacturing of a linear motor employing a wiring harness incurs additional cost of material and assembly labor. Therefore, it would be desirable to eliminate the use of a wiring harness in a linear motor to decrease the cost of assembly, decrease the overall weight of the moving element, and to eliminate the clearance restrictions on the linear motors utility.
Most linear motors are manufactured to follow a straight path and to be of a predetermined fixed length. This establishes the length of the armature, and consequently the number of armature windings. In such linear motors, all armature windings lie parallel to each other, with axes thereof generally 90 degrees to the travel direction of the linear motor. In order to make a new linear motor of any specific length, a new assembly must be tooled. Each assembly has a set number of armature windings, a set number of moveable magnets, and, a fixed length wiring harness associated with the moveable element of the linear motor. The cost of producing a linear motor is increased because each assembly must be custom designed to a users needs, with new tooling required for each such design. Therefore, it is particularly desirable to produce a linear motor of a modular design.
A modular designed motor would allow easy customization for any desired length armature winding assembly. The cost of manufacturing a particular linear motor would be decreased since the cost of tooling would be minimal. A data base of assembly and outline drawings will be common to all assemblies within a family of linear motors, easing assembly and manufacturing. A stocking of common parts would allow quick assembly of any special length motor assembly, from now readily available parts. The stocking of common parts also decreases overall cost of manufacturing since materials will be bought in bulk from common suppliers. The assembly of any desired length armature winding assembly will enjoy a decreased lead time. As such, a modular designed linear motor provides for a decrease in manufacturing cost, decrease in lead time to assemble, and increases overall utility.
Linear motors using a series of stationary armature windings and moving magnets require a means to dissipate heat from the coils. Linear motors having cold plates mounted on one edge of an armature winding are known in the art. Alternatively, armature windings having cooling coils or channels are also well known in the art. Examples of such armatures are disclosed in U.S. Pat. No. 4,839,545. These armatures use stacked laminated magnetic material.
Linear motors having non-magnetic armatures are also known, an example of which is disclosed in U.S. Pat. No. 4,749,921. The linear motor of the referenced disclosure has a non-magnetic armature which includes a coil support structure composed of an aluminum frame or a serpentine cooling coil. In the embodiment having an aluminum frame, heat is carried away from the coils of the armature via the aluminum frame and a side plate which functions as a heat sink. Alternatively, a serpentine coil may be employed to effect more uniform cooling within the armature. The serpentine coils support the overlapping coils while the coils and the armature are cast in a block of settable resin. However, the incorporation of such a coil has the disadvantage of increasing costs because of the complexity of assembly and material expenses. Furthermore, while the use of the settable resin prevents the occurrence of eddy currents, the thermal conductivity of the settable resin is significantly less than that of metals which it replaces and thus reduces the power dissipation capacity of the linear motor.
Linear motors are increasingly being employed in manufacturing equipment. In such equipment, nominal increases in the speed of operation translate into significant savings in the cost of production. Therefore, it is particularly desirable to produce as much force and acceleration as possible in a given linear motor. An increase in force generated requires either an increase in magnetic field intensity or an increase in current applied to coils of the armature. In a permanent magnet linear motor, the available magnetic field intensity is limited by the field strength of available motor magnets. Power dissipated in the coils increases at a rate equal the square of the current. Attendant heat generation limits the force that may be achieved without exceeding the maximum armature temperature. Therefore, improvements in the power dissipation capacity of linear motors provide for increases in their utility.
The present invention relates to a magnet configuration for a linear motor. According to one aspect, a magnet configuration for a linear motor includes an array of a plurality of magnets having alternating polarities and arranged to define a span. Additional magnets are positioned outside of the span near opposed ends of the span. Each of the additional magnets has a magnetic polarity opposite that of an adjacent one of the plurality of magnets, which is located at a corresponding end of the span. As a result of such magnet configuration, the array of magnets provides a substantially sinusoidal magnetic field in the span.
Another aspect of the present invention provides a linear motor system that includes a stage having a plurality of motor magnets arranged in an array having first and second ends that are spaced apart from each other. Additional magnets are positioned outside of the array adjacent respective motor magnet at the first and second ends of the array to help the array of motor magnets provide a substantially sinusoidal magnetic field. A plurality of armature windings defines a path along which the stage may traverse. A plurality of switches are associated with the path, with each switch being associated with a respective winding. Each switch is operative to energize an associated armature windings so as to urge the stage along the path. For example, the switch may be responsive to one or more switching magnets located on the stage, so that the armature winding associated with the switch can be energized when the switch is within the influence of a switching magnet of the stage.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.